Sheet metal blanks are deep drawn into various shapes to form cases for use in a number of different applications. According to conventional deep drawing techniques, a blank is clamped in place over a die and a punch pushes the blank through an opening in the die to form the case. The depth of the case that can be formed in a single drawing operation depends on the tensile strength and thickness of the metal. In general, one or more redrawing operations are necessary to obtain a case with the desired depth and size.
In a specific application, blanks are drawn to form cylindrical battery cases. For example, casings for small size batteries (e.g., AA, AAA) have a relatively long side wall and a relatively small diameter. Because of the depth and small diameter, a blank typically undergoes one drawing and at least one redrawing operation to obtain the desired dimensions. Therefore, it is desirable to have a method and apparatus which performs all of the necessary drawing operations to form a blank into the desired shape and size.
Previous methods and apparatus for performing multiple drawing operations involve the use of a deep drawing transfer press. The deep drawing transfer press carries out an in-line process during which workpieces are transferred through a series of work areas. Transfer presses typically have a beam which carries a plurality of punches. The punches are aligned with matching dies so that when the beam is lowered, the punches pass through the corresponding dies. The drawing means are shaped to perform different drawing operations and are arranged so that the last set forms the desired shape.
Unfortunately, the use of a deep drawing transfer press overly limits the rate at which cases are produced. In operation, conventional transfer presses typically load a blank at a first work area corresponding to the first drawing means. The beam is then lowered so that the first punch forces the blank through the first die to perform a first drawing operation. After the first drawing operation is completed, the workpiece is transferred to a second work area for performing a second drawing operation with the second drawing means, and so on. It will be appreciated that each workpiece must dwell at each work area so that the drawing apparatus can perform the associated operation. As a result, techniques using a deep drawing transfer press proceed in a step-by-step fashion which limits the rate at which cases are formed. Accordingly, typical transfer presses have a maximum production rate of approximately 170 parts per minute. Furthermore, it will be appreciated that when the beam is lowered, it performs a number of different drawing operations simultaneously. The discrete repetition of simultaneous drawing operations creates a significant noise problem.
Deep drawing transfer presses have additional problems in transferring each workpiece through the work areas. Transfer presses typically use fingers which clamp onto the workpiece for transfer to the next work area. The workpieces are held until the next punch pushes the pieces out of the fingers while performing a drawing operation. The fingers may interfere with the drawing apparatus associated with the work area and therefore are often broken, requiring downtime for repair or replacement. In addition, the fingers may disrupt the drawing procedure, causing additional delays or slower operation of the press.